Method of determining resistivity of a thin layer



1968 R. J. BONCUK ETAL 3,416,073

METHOD OF DETERMINING RESISTIVITY OF A THIN LAYER Filed July 8, 1965(:jjj/i/m 23/ Fig.l

Richard. J. Han k Albert E. Ozia r.

ATrS's.

United States Patent Oce 3,416,078 Patented Dec. 10, 1968 3,416,078METHOD OF DETERMINING RESISTIVITY OF A THIN LAYER Richard J. Boncuk,Scottsdale, and Albert E. Ozias, Jr.,

Tempe, Ariz., assignors to Motorola, Inc., Franklin Park, Ill., acorporation of Illinois Filed July 8, 1965, Ser. No. 470,377 11 Claims.(Cl. 32464) ABSTRACT OF THE DISCLOSURE A method of determining theresistivity of a thin layer of electrically conductive material whichincludes applying a wide area ohmic contact directly to the thin areaand in physical contact therewith. Next, a conductive circuit includingthe thin layer and the contact is completed in order to apply a currentof known value to the contact. By measuring the induced voltage dropacross the thin layer, the resistivity thereof can be determined.

This invention relates to a method of determining the resistivity ofelectrically conductive thin layers on electrically conductive materialsor substrates and more particularly to electrically conductive thinlayers on semiconductor materials.

The term electrically conductive materials is intended to include themany elements and compounds which are capable of the transmission ofelectricity. These elements and compounds are commonly classed in thetwo broad categories of conductors, such as copper and aluminum, andsemiconductors, such as silicon and germanium.

In the field of semiconductor devices there is a desire to build unitsas small as possible. This miniaturization is limited by availableproduction techniques and methods by evaluating the products during thevarious steps of manufacturing. One of the production techniques whichhas aided this trend toward smaller devices is the well known epitaxialprocess which allows the depositing of a thin layer of closelycontrolled thickness and resistivity. In the evaluation of this thinlayer many methods have been available for determining the thicknessrapidly and accurately but, there has not been a suitable method ofaccurately determining the resistivity of multilayer semiconductormaterials of the same conductivity type.

The resistivity of a semiconductor material is commonly measured inohm-centimeters. The resistivity is directly related to theconcentration of impurities in the semiconductor material, and thereforethe ascertainment of the resistivity is important in the determinationof the processing steps required for the semiconductor material so thatdevices of desired specifications may be produced.

The four-point probe, which is commonly used to measure the resistivityof semiconductor materials, does not provide accuracy in the measurementof the resistivity of thin layers deposited on substrates of similarconductivity type because the spreading resistance effect requires theprobes to be placed impractically close together, on the order of thethin layer thickness apart or less, and the tendency of the lowresistivity substrate to act as a short.

The three-point probe method relies on the breakdown voltagecharacteristics of the material at one of the probe contact points toyield a figure which may be used to calculate the resistivity. Thisthree-point probe method requires skilled judgment as to when thebreakdown occurs and the results are only approximations of the actualresistivity.

The resistivity may also be determined by the measurement of thebreakdown voltage of a diode formed on the thin layer and the indirectcalculation from the measured breakdown voltage of the resistivity.Another method is to calculate the resistivity from the measuredcapacitance of a diode formed on the thin layer. These methods yieldonly indirectly calculated approximations of the resistivity andtherefore include considerable possibility of error being introducedinto the process.

It is an object of this invention to provide a method of accuratelydetermining the resistivity of a thin layer on an electricallyconductive material.

It is another object of the invention to provide a rapid andreproducible method of determining the resistivity of a thin layer on asurface of an electrically conductive material.

A further object of this invention is to provide a method of determiningthe resistivity of a thin layer on an electrically conductive materialwherein the preparation of the material is relatively simple.

A still further object of this invention is to provide a method ofdetermining the resistivity of a thin layer deposited on a semiconductormaterial of similar conductivity type.

A feature of this invention is a novel method of determining theresistivity of a thin layer on a substrate which utilizes area contactsof a very low resistance, commonly called ohmic contacts, formed on thesurface of the thin layer and the substrate.

Another feature of this invention is a method of determining theresistivity of a thin layer on a substrate in which the ohmic contactsused are of sufiicient area that the spreading resistance etfect is anegligible factor in the determination of the resistivity.

A further feature of this invention is a method of determining theresistivity of a thin layer on a substrate in which independent circuitloops are used for applying a current and measuring the induced voltagedrop.

In the accompanying drawing:

FIG. 1 is a schematic view showing an arrangement for makingmeasurements to determine resistivity according to the presentinvention; and

FIG. 2 is a schematic view showing an arrangement for makingmeasurements to determine resistivity according to a second embodimentof the present invention.

The determination of the resistivity of a thin layer on a substrate, inaccordance with this invention, includes the forming of spaced ohmiccontacts on the thin layer and the substrate with at least one of thecontacts being on the thin layer and the contacts having an areasignificantly larger than a point contact. A substantially constantcurrent of known value is applied to the contacts and the inductedvoltage drop is measured. The voltage drop measured together with theknown current, the contact areas and the layer thickness enablesdetermination of the resistivity of the thin layer.

In accordance with this invention, accurate and reproducibledetermination of resistivity is obtainable for thin layers over a broadrange of resistivity values, with values, depending upon the resistivityof the substrate, as low as about 0.05 ohm-centimeters and an apparentlyunlimited upper value. The normal range of resistivities measured isapproximately 0.3 to 150 ohm-centimeters. Accurate measurements usingthe method of the invention are achieved when the substrate upon whichthe thin layer is deposited advantageously has a lower resistivity thanthe thin layer and is of similar conductivity type.

The thin layers which may be measured to determine the resistivity maybe formed by one of the well known processes such as the epitaxialprocess which are used to form thin layers, e.g., thicknesses in therange between about /2 micron or less to microns or more. Thedetermination of the resistivity according to this inventionadvantageously is used for thin layer thicknesses of about 5 microns to50 microns although this method may be used on thicker material when asuitable conductive backing is supplied.

The method of determining the resistivity according to this inventionincludes the measurement of the induced voltage drop that results when aconstant current is passed through the thin layer. The voltage drop thatnormally results from contact resistance is minimized by the use of ahigh impedance voltmeter having a minimum input impedance of about 1megohm and by the formation of large ohmic contacts, of a sizesignificantly larger than a point contact, on the surface of the thinlayer and the substrate. A high impedance voltmeter utilizes anegligible portion of the current in a circuit and thereby yieldsreliable measurements while exerting an insignificant effect.

The ohmic contacts used may be formed by any of the rnany methods wellknown in the semiconductor art. A preferred method, is the evaporationof a compatible metal onto the thin layer and the substrate and asubsequent alloying of the metal with the thin layer and the substrate.

Advantageously, the contacts should be of an area greater than thesquare of the thickness of the thin layer to be measured; however, it ispreferred to have the size of these contacts somewhat larger than thisminimum size, at least an order of magnitude. These contacts may becircular, rectangular, or any other geometrical shape which produces acompact contact and is conveniently formed. The use of contacts of thissize reduces the voltage drop resulting from the spreading resistanceeffect to a negligible amount.

The contacts used for measuring resistivity according to this inventionadvantageously are spaced at least 8 mils and preferably spaced betweenabout 8 and 50 mils with the minimum distance determined by thecapabilities of the facilities to form the contacts and the upper limitbeing determined by the resistivities and the thicknesses of the thinlayer and the substrate.

The substantially constant current that is applied to these contacts mayvary over a broad range, for example, from about 0.1 to 30 amps persquare centimeter. The maximum current is limited to an extent by thesize of the contacts and the point where the material breaks down. Theuse of current densities below about 1 amp per square centimeter hasyielded accurate resistivity determinations on a wide range of thinlayers.

The following explanation of the determination of the resistivity ofsemiconductor material in accordance with this invention will bedescribed with reference to the accompanying drawing. In FIG. 1, whichshows one embodiment of this invention, an epitaxial layer 10 of P dopedsilicon, about 11 microns thick, was deposited on a P+ doped siliconsubstrate 12. Two contacts 17 of aluminum were deposited by evaporationon the surface of the epitaxial layer 10 and alloyed to the epitaxiallayer 10 by heating. A very low resistance contact, commonly called anohmic contact, was obtained on the surface of the material to bemeasured by this metal alloying process. The method of forming thismetal alloy may be one of th many processes well known in thesemiconductor art. When the size of the contact area is selected aspreviously described the spreading resistance effect is a negligibleportion of the measured voltage drop. In this embodiment, the contacts17 were each about 15.6 x 10- square centimeters and about 8 mils apart.

Probes 20 from a constant current source 22 were connected to each ofthe contacts 17 on the epitaxial layer 10 and a constant current ofknown value was passed through current circuit loop 23. A currentdensity of 0.577 amp per square centimeter was applied to the contacts17 in this embodiment.

Probes 25 from a high impedance voltmeter 27 with an input impedance of10 megohms were connected through a voltmeter circuit loop 28, which wasseparate from the current circuit loop 23, to the contacts 17 and thereading of the voltmeter 27, in conjunction with the other known values,was substituted into the formula to calculate the resistivity directly.In this expression, r is the resistivity, V is the measured voltage, Iis the substantially constant current, A is the area of one contact onth thin layer or an average of the areas of the two contacts if they areunequal, and L is the epitaxial layer thickness.

The thickness of the epitaxial layer 10 was determined by a BeckmanCsBr-IR-SA instrument using infra-red light reflection although any ofthe other methods well known to the art, such as bevel and stain orweight differential, may be used to determine this layer thickness.

In the above embodiment it is believed that the current flows throughthe epitaxial layer 10 in effectively straight lines to the substrate.Therefore, the resistance which causes the voltage drop bears a directrelationship to the area of the contacts 17 and the thickness of theepitaxial layer 10. The voltage drop in the substrate 12 may bedisregarded because the resistance of this material is negligiblecompared to the epitaxial layer 10.

In another embodiment of this invention, FIG. 2, a single aluminum ohmiccontact 40, of known area, was deposited on an epitaxial layer 42 and alow resistance backing 43 of gold gallium was evaporated and alloyed onthe opposite side of the substrate 44. The substrate 44 was placed on acopper vacuum chuck 45 so that the gold gallium backing 43 was inintimate ohmic electrical contact with the chuck 45. The probe 46 from aconstant current source 48 which has one lead 49 connected by solder 50to the copper chuck 45 to form a current circuit loop 51, was connectedto the contact 40 and a constant current of known value was passedthrough the loop 51. The probe 52 from a high impedance voltmeter 53having an input impedance of 10 megohms which had one lead 54 connectedby solder 55 to the copper chuck 45 to form a voltmeter circuit loop 56separate from the current circui-t loop 51, was connected to the contact40 and the induced voltage drop was measured. The reading from thevoltmeter 53, in conjunction with the other known values, wassubstituted into the formula to calculate the resistivity directly, thesymbols being as defined above.

In this embodiment it is believed that the current flows through theepitaxial layer in effectively straight lines and therefore the formulafor the calculation of the resistivity has been adjusted to reflect thevoltage drop caused by the single passage of current through theepitaxial layer. The voltage drop in the substrate may be disregardedbecause the resistance of the subtrate is negligible when compared tothe epitaxial layer.

This invention thus provides a simpler and more accurate method ofdetermining the resistivity of thin layers on substrates, especially ofthin layers on semi-conductor materials, than was previously available.

We claim:

1. A method of determining the resistivity of a thin layer ofsemiconductor material of relatively high resistivity and formed on asubstrate of relatively low resistivity including the steps of: applyingan ohmic contact to said thin layer, said contact being in directphysical contact with the thin layer and having an area significantlygreater than a point contact, completing a conductive circuit whichincludes said contact, said thin layer and said substrate, applying acurrent of known value through said conductive circuit, thereby passingsaid current through said conductive circuit and measuring the inducedvoltage drop across said thin layer in order to determine theresistivity thereof.

2. The method defined in claim 1 wherein said contact is formed byevaporating a metal on said thin layer.

3. The method defined in claim 1 wherein said contact has an areagreater than the square of the thin layer thickness.

4. The method defined in claim 1 wherein the step of completing aconductive circuit includes applying another contact to the surface ofsaid thin layer and spaced apart from said first applied contact, andmeasuring the induced voltage drop in the thin conductive layer beneaththe contacts.

5. The method defined in claim 4 which includes spacing said anothercontact approximately 8 mils apart from the first formed contact andpassing a current having a density of between approximately 0.1 and 30amperes per square centimeter through the conductive thin layer andbetween the contacts.

6. A method of determining the resistivity of a thin layer ofsemiconductor material of relatively high resistivity and formed on asubstrate of relatively low resistivity which includes the steps offorming an ohmic contact directly on the thin layer and in physical andelectrical contact therewith, forming a conductive path which includessaid contact, said thin layer and said substrate and which is adapted-to receive an energizing potential for passing current through saidconductive path, and applying a current of known value through saidconductive path so that said current passes through said conductive pathand measuring the induced voltage drop across the thin layer in order todetermine the resistivity thereof.

7. The method defined in claim 6 which further includes forming a layerof gold gallium on the surface of the substrate opposite to that surfaceupon which the thin semi-conductor layer is formed, attaching a coppervacuum chuck to the layer of gold gallium, said current being passedthrough said thin layer, through said substrate, through said layer ofgold gallium and to said copper vacuum chuck, and measuring the inducedvoltage drop between said copper chuck and said ohmic contact on saidthin layer to thereby determine the resistivity of said thin layer.

8. The method defined in claim 6 wherein the step of forming aconductive path includes forming another ohmic contact on the surface ofsaid thin layer and spaced apart from the first formed ohmic contact,said current being passed between said ohmic contacts and through saidthin layer beneath the contacts, and measuring the induced voltage dropacross the thin layer beenath the contacts in order to determine theresistivity of the thin layer.

9. The method defined in claim 8 wherein both contacts on the surface ofsaid thin layer are formed with an area greater than the square of thethin layer thickness, and said contacts on the thin layer are spacedapart between approximately 8 mils and mils on the thin layer.

10. The method defined in claim 9 which includes passing a currenthaving a density between approximately 0.1 and 30 amperes per squarecentimeter through said thin layer.

11. A method of determining the resistivity of a thin layer ofsemiconductor material of relatively high resistivity and formed on asubstrate of relatively low resistivity which includes the steps offorming an ohmic contact directly on the thin layer and in physical andelectrical contact therewith, forming a layer of gold gallium on thesurface of the substrate opposite to the surface upon which the thinsemiconductor is formed, attaching a copper vacuum chuck to the layer ofgold gallium, passing a current between said contact and said copperchuck, and measuring the induced voltage drop between said ohmic contactand said copper chuck to thereby determine the resistivity of said thinlayer.

References Cited UNITED STATES PATENTS 2,921,257 1/1960 Boicey 324-2,142,619 1/1939 Sciaky 324--64' 3,287,637 11/1966 Keller 324-61 XROTHER REFERENCES Kovacs et al., Determination of Impurity DistributionProfiles in Silicon Epitaxial Wafers, SCP and Solid State Technology,August 1964, pp. 32-36.

Semiconductor Resistivity Test Set, Baird Associates Inc., TechnicalBulletin TP-lO, February 1956, 4 pages.

RUDOLPH V. ROLINEC, Primary Examiner.

E. E. KUBASIEWICZ, Assistant Examiner.

